Optical devices such as optical waveguides can be fabricated in Silicon-on-Insulator (SOI) films in configurations that are compatible with integrated circuit structures. One promising implementation, which has been described elsewhere, involves the use of a single crystal SOI film and the formation of a thin poly-silicon upper layer deposited on the SOI film. In this implementation, the thin poly-silicon upper layer is patterned to form the light guiding element or patterned with the SOI film to form SOI/poly composite light guiding features. Poly-silicon alone can also guide light. However, the additional crystalline silicon is desirable to minimize optical losses.
Unfortunately, modern silicon etch systems and processes are optimized so as to prepare vertical features that closely match the dimensions of the masking film. Such vertical features are efficient and necessary for fabricating poly-silicon gates for advanced microelectronics, but the sharp edges of the vertical features can degrade performance in optical device structures such as optical waveguides. Also, patterning using these silicon etch systems and processes can contribute to irregular edges, especially when applied to polycrystalline films.
An optical device according to one embodiment of the present invention has an upper silicon or poly-silicon feature with rounded corners. Rounding the corners of the upper silicon poly-silicon feature improves the performance of optical devices such as optical waveguides.